There are wireless communications systems which facilitate wireless communications between a wireless base transceiver station (BTS) and terminals by using the Orthogonal Frequency Division Multiplexing (OFDM) scheme (see, e.g., Reference 1).
In a downlink from the wireless BTS to a terminal in the OFDMA (OFDM Access) system, the wireless BTS allocates transmission data to frequency subbands of better wireless quality, thereby improving throughput.
In order for the wireless BTS to allocate the transmission data to a better wireless quality subband in a frequency domain, the terminal informs the wireless BTS on the wireless quality of the frequency subbands of the downlink through an uplink from the terminal to the wireless BTS.
In this case, if the terminal informs the wireless BTS on the wireless quality for all the frequency subbands of the downlink, the uplink can be congested. Therefore, there has been suggested a method, wherein the terminal informs the wireless BTS on only a few frequency subbands of wireless quality in the downlink except for those of poor wireless quality.
Further, CQI (Channel Quality Indicator) information is used as the wireless quality information.
FIG. 3 describes an example of the method for reporting top n CQIs from the terminal to the wireless BTS, wherein n is an integer equal to or greater than 1.
In this example, CQIs of five frequency subbands A, B, C, D and E are detected from a single terminal and the three top CQIs, i.e. the CQIs of the subbands B, C and D are reported to the wireless BTS. The CQIs of the subbands A and E are not reported to the wireless BTS.    [Reference 1] Japanese Patent Laid-open Publication No. 2006-020156    [Reference 2] 3GPP TS 36.211 V1.1.0 (May 2007)
However, in the system of reporting the wireless quality (CQI) from the terminal to the wireless BTS as described above, if the wireless quality of poor wireless quality frequency subband is not reported even one time, the quality of that frequency subband is totally unknown to the BTS. Further, if a frequency subband whose wireless quality was good and thus reported to the BTS suffers from an abrupt quality degradation, the wireless quality thereof will not be reported due to its poor wireless quality and may still be regarded as a good wireless quality frequency subband. Therefore, the wireless BTS cannot perform proper scheduling in the frequency domain, which results in a lower throughput.